In SAS communications one or more SAS initiator devices may be coupled to one or more SAS target devices through zero or more SAS expanders to form a SAS domain. In general, SAS initiator devices may initiate or request an initial connection to a particular identified SAS target device. Prior to initiating or requesting such a connection, devices within the SAS domain must perform initial processing to establish one or more mutually supported speeds for communication between any pair of connected devices. In general, each SAS device (initiator, target, or expander) may support one or more SAS speeds for communication with other SAS devices coupled to its PHYs. SAS specifications provide for a variety of SAS speeds standardized for communications between SAS devices. Some devices may support only one or a few lower speeds, other devices may support only one or a few higher speeds, and many devices support a broad spectrum of high and/or low SAS speeds.
The process of establishing the mutually supported speeds that may be used for communication between any two devices is referred to as speed negotiation. In the currently evolving SAS-2 specifications, each device must support processing during a speed negotiation window. The speed negotiation process in the specification defines signals exchanged between a pair of SAS devices to determine one or more SAS speeds supported by both devices in a particular desired link. If multiple speeds are supported by both devices, the highest supported speed is generally utilized if performance is critical while a slower speed may be utilized if reliability is more important than communication speed.
As presently specified in the evolving SAS-2 specifications, processing during the speed negotiation window exchanges signals between a pair of SAS devices and simply identifies all the potential speeds that are supported by both devices. In addition to the SAS speed, the evolving SAS-2 specifications specify a number of speed options that may be modified in support of each SAS speed. These speed options include, for example, whether or not spread spectrum clocking is supported for each SAS speed, the type of spread spectrum clocking supported, and the logical link rate requested in support of SAS-2 multiplexing. The SAS-2 specifications and other related background material generally known to those of ordinary skill in the art is readily available at www.t10.org and other public locations and sites.
Interoperability of SAS devices from various vendors is an ongoing challenge. In particular, in view of the various speed options that may be specified for each SAS speed, the challenge of interoperability is further exacerbated. As presently specified in the evolving SAS-2 specifications, there is little assurance that a particular SAS speed, though mutually supported by both devices, will provide reliable exchanges between the pair of devices. One vendor may utilize particular default speed options settings while another vendor may prefer other default speed option settings for the same SAS speed. Nothing in the present evolving version of the SAS-2 specifications provides for relief from this interoperability challenge.
It is evident from the above discussion that an ongoing need exists to improve reliability of transfers between a pair of SAS devices by assuring that a mutually supported SAS speed will provide a desired level of both performance and reliability.